1. Field of the Invention
The invention relates to a femoral neck endoprosthesis for an artificial hip joint, consisting of an adapter for accommodating the joint ball, a support ring for axially and radially guiding the adapter on the proximal side of the femur, and a support bearing for guiding the adapter on the lateral side of the femur.
2. Discussion of the Prior Art
Such femoral neck endoprosthesis are already known in a wide variety of forms.
For example, DE 27 24 234 A1 discloses such a prosthesis. The adapter accommodating the joint ball is provided with a support shoulder, and a shaft protrudes far into the femur. The approximately conical support shoulder sits on a support ring which is fixed on the femoral neck by the form of its outer edge. On the lateral side of the femur, a support bearing covering a large surface area is screwed on from the outside.
Through this support bearing, a long screw with a long shaft which penetrates the femur is fixed laterally to the adapter by means of a thread. This screw has a long guide section which sits with a form-fit in a corresponding long bore in the shaft of the adapter. This long screw braces the adapter at one end against the support bearing and at the other end against the support ring. Without this axial bracing, the adapter is not able to reliably introduce the considerable loads into the femur.
Such a prosthesis has a number of disadvantages. The large surface areas of the cortical substance enclosed by parts of the prosthesis are no longer adequately supplied after the prosthesis has been fitted. The properties of these areas of the cortical substance alter in an adverse manner. The cortical substance can no longer take up the forces which normally occur. The functional capacity of such a prosthesis is thus considerably limited in time.
The permanent bracing of the femur between support bearing and support ring leads to a permanent deformation of the bone and therefore to a loosening of the prosthesis. The permanent loading of the bone as a result of said bracing leads to damage of the cortical substance, above all at the pressure surfaces. The resistance of the cortical substance diminishes. After a relatively short time, the functional capacity of the prosthesis in no longer guaranteed.
A similar prosthesis is also described in EP 207 985 B1. The support body assigned to the adapter is in this case configured as a prismatic body which is introduced with a form-fit into a corresponding recess prepared in the femur. This prismatic support body ensures that the forces normally acting on the prosthesis via the joint ball are transmitted into the approximately perpendicular parts of the cortical substance over a relatively large surface area and at an optimum angle of transmission.
However, a disadvantage in this case is once again that the prismatic support body can only fulfill its function if it is braced against a support bearing on the other side, i.e. the lateral side, of the femur by means of a screw connection.
The disadvantages described earlier are also present in this design. In the case of this solution, a further considerable disadvantage lies in the great loss of bone substance. Under the effect of the normal loads and the necessary bracing force, the wedge-shaped support body leads to additional deformation of the femur.
The rigid connection between the support bearing and the adapter via a long fitted sleeve causes additional damage to the femur. The useful life of such a prosthesis is thus likewise considerably limited.
A further alternative design was proposed in a thesis by Mr Garnal Baroud at the Technical University of Chemnitz (defended on Dec. 12, 1997). The adapter is in this case likewise provided with a support shoulder. At its outer end, the support shoulder merges into a long shaft which extends to the other side of the femur and which preferably has a limited elasticity.
The adapter of said configuration is introduced into a guide sleeve surrounding the shaft and the support shoulder. This guide sleeve consists of a support ring, a support bearing, and a sleeve-like elastic connection between the two support elements.
The support ring has, in a rotationally symmetrical recess, an axial stop for the support shoulder and a bore for the shaft of the adapter. Provided at the other end of the guide sleeve is the so-called support bearing which is held with a radial form-fit in a corresponding bore in the lateral cortical substance. The support ring and the support bearing are connected to each other with limited elasticity via the sleeve-like, tightly wound spiral spring of relatively large diameter.
This assembled guide sleeve is introduced from the proximal side into a correspondingly large bore in the femur. The radially directed flange of the support ring lies, transversely with respect to the axial direction of the guide sleeve, on a correspondingly worked bearing surface of the cortical substance.
The adapter which is then fitted is held and radially guided in this guide sleeve with its support shoulder on the stop surface in the support ring.
By means of this arrangement of the guide sleeve and the limited elasticity of the shaft of the adapter, and also by means of a spring washer which can be inserted, if necessary, between the adapter and the guide sleeve, movements within the prosthesis and possible peak loads can be taken up and transmitted into the femur over a large surface area.
A considerable disadvantage has proven to be that under these conditions a relatively large proportion of the forces acting transversely with respect to the axis of the adapter are transmitted into the relatively soft spongy substance of the femur, at least until such time as the prosthesis has become incorporated. The primary stability of the prosthesis is thereby limited.
A further disadvantage lies in the relatively high production outlay and the relatively large amount of foreign material which has to be introduced into the femur. Another negative factor is that a relatively large part of the femur has to be removed. However, the decisive disadvantage lies in the required connection between the support ring and the support bearing. The initial press-fit of the support bearing in the lateral cortical substance and the later incorporation of this support bearing lead, in this embodiment too, to stresses in the femur and at the contact points between the femur and the prosthesis.
These stresses still lead to damage of the bone tissuexe2x80x94albeit to a far lesser extent than in the prostheses described in the introductionxe2x80x94and therefore contribute to reducing the useful life of the prosthesis.
It is therefore an object of the present invention to make available a femoral neck endoprosthesis which avoids bracing the femur for the purpose of securing the elements of the prosthesis and during normal loading. The invention is intended to create conditions in which local peak loads can be reduced within the prosthesis body.
The contact surfaces between the prosthesis and the supporting parts of the cortical substance are to be configured in such a way that the natural supply to the bone tissue is guaranteed during the required period of functioning of the prosthesis.
The inventive femoral neck endosthesis includes an adapter for accommodating the joint ball, a support ring for axially and radially guiding the adapter on a proximal side of the femur, and a support bearing for guiding the adapter on a lateral side of the femur. The adapter has a support shoulder directed toward the femur and a shaft which penetrates the femur approximately along an extended axis of the femoral neck. The support ring is mounted only on the proximal side of the femur and has a flange support surface mounted on a worked bearing surface of the proximal cortical substance of the femur and a hub mounted inside the femur. The support ring further has an axial stop for the support shoulder of the adapter and a bore for the shaft of the adapter so as to guide the adapter. The bore is laterally offset in relation to the support surface. The support bearing is fixed only on the lateral side of the femur, substantially on the lateral cortical substance. The support bearing has a guide bore for the shaft of the adapter, which guide bore has an active access that is lineable with the axis of the adapter.
As a result, in the end, of the absence of a rigid connection between the support ring and the support bearing, loads are introduced into the proximal cortical substance, in the same way as under natural conditions. Bracing of the femur transversely with respect to its longitudinal axis is completely avoided. The forces are predominantly introduced into the stable proximal cortical substance. Even under unfavorable load conditions, none of the feared tensile forces occur at the contact points between the prosthesis and the femur. The narrow contact surfaces can be reached at all times by the body fluids supplying the bone. Disintegration of the bone structure can therefore be avoided for much longer.
Because of the omission of a mechanical connection between the proximal and lateral cortical substance, the femur as a whole can deform in a natural way. This contributes considerably to preserving the functions of the bone tissue.
In view of the tendons and muscles present in the human body in this area, it is not necessary to provide additional axial securing of the adapter in the support ring and in the support bearing against movement in the proximal direction.
The prosthesis is easy to manufacture and can also be adapted without difficulty to different sizes and shapes of femur.
The prosthesis possesses the necessary primary stability.
The configuration of the support surface of the adapter and of the support ring with a suitable central cone angle ensures that the load normally applied via the joint ball can be introduced under optimum conditions into the supporting cortical substance. In the event of excessive loading, additional safety is afforded by the bore in the support ring.
The support surface of the support ring on the proximal cortical substance permits optimum introduction of the normal load along the femur. The support ring itself is elastic in the area between the support shoulder of the adapter and the bearing surface of the cortical substance. It can therefore also reduce the unavoidable peak loads.
In addition to the desired elasticity, the support ring can have a substantially nonelastic inner conical flange and an axially elastic spring ring between the support shoulder of the adapter and the inner cone of the conical flange to ensure a certain damping of peak loads as forces are introduced into the femur.
In another embodiment of the invention the support bearing penetrates the lateral cortical substance of the femur approximately perpendicular to its surface and with a form fit. The support bearing furthermore has a bore for guiding the shaft of the adapter. The axes of the bore is inclined in relation to the axis of the support bearing by an angle between 5xc2x0 and 30xc2x0. In yet another embodiment the support bearing is provided with profiles for turning the support bearing in the cortical substance. The bore is aligned with the probable position of the axis of the shaft of the adapter and has a dome shape. These embodiments of the support bearing permit a low-weight design of the support bearing with a very small contact surface against the supporting cortical substance. The cortical substance is adequately supplied as far as the center of the bearing surface. The axis of the bore guiding the shaft of the adapter can in this case be aligned from the outset to the desired angle. The dome shape of the bore permits a certain correction in situ.
In yet another embodiment the support bearing consists of an outer guide ring with a spherical inner guide surface, and a spherical slide ring which can be fitted in the guide ring and which has a bore for the shaft of the adapter. This design of the support bearing permits adaptation of the bore in a greater range with optimum mobility of the shaft in the bore.
In still another embodiment a detachable axial bearing is assigned to the outer end of the shaft. The axial bearing can be a threaded bushing whose external diameter penetrates the bore in the support bearing with play. The threaded bushing has axially directed stops on the outside and can be axially adjusted at the end of the shaft by a screw connection and can be fixed in the adjusted position. This arrangement of the axial bearing and its design ensure that the functional capacity of the prosthesis is maintained even under extreme conditions. However, it is important here that adequate play is at all times present between the axial stops of the bushing and the support bearing and that the bushing does not come away from the shaft.